The present disclosure relates to the subject matter contained in Japanese Patent Application No.283734 filed on Sep. 10, 2002, which are incorporated herein by reference in its entirety.
1. Field of the Invention
The present invention is related to an electrophotographic type image forming apparatus such as a copy machine, a printer, and a facsimile, and also related to a fixing apparatus thereof. Precisely speaking, the present invention is directed to a simultaneous image-transferring/fixing type image forming apparatus and a fixing apparatus thereof.
2. Description of the Related Art
Conventionally, image forming methods have been widely utilized by which electrostatic latent images are formed on photosensitive drums, these electrostatic latent images are developed by using dry type toners, and thereafter, the toner images are transferred onto recording media in electrostatic manners so as to be fixed thereon. In these conventional image forming methods, since concave/convex portions of a surface of paper corresponding to the recording media are present, this paper cannot be completely made in close contact with such a photosensitive drum, so that ununiform gaps may be produced. Therefore, there is such a problem that a transfer electric field is disturbed. Also, since Coulomb""s repulsion may occur among the toners, there is another problem that images are disturbed.
To solve this problem, an image forming method of forming a color copy in the below-mentioned manner, and another image forming method has been proposed. That is, when a plurality of toner images having different colors are transferred onto an intermediate transfer member in a multiple manner, an electrostatic field is utilized, whereas when a multi-colored toner image transferred onto the intermediate transfer member is again transferred onto a recording medium, heat is utilized in order that the melted multi-colored toner image on the intermediate transfer member is transferred onto the recording medium and, at the same time, is fixed on this recording medium, and thus, such a color copy may be produced. On the other hand, in the latter image forming method, while a toner image formed on an endless belt-shaped photosensitive drum is heated so as to be melted, the melted toner image is transferred onto a recording medium so as to be fixed thereon.
In these image forming methods, since the toner images are transferred onto the recording media, these image transfer operations are carried out by utilizing heat in a non-electrostatic manner, deteriorations of image qualities which are caused by the disturbance of transfer electric fields and the Coulomb""s repulsion can hardly occur.
Furthermore, in the above-described non-electrostatic image forming methods by such a simultaneous image transferring/fixing method, U.S. Pat. No. 2,990,278, JP-A-5-19642, JP-A-5-107950, and JP-A-5-249798 have described the following methods. That is, in order to transfer a toner image formed on an image carrier such as an intermediate transfer member and a photosensitive drum onto a recording medium under better condition, while the image carrier is made in close contact to a recording medium, heat and pressure are applied to this image carrier. Thereafter, both the image carrier and the recording medium are cooled while this image carrier is made in contact with the recording medium, so that a toner image sandwiched between the image carrier and the recording medium is made solid, and also, this solidified toner image is fixed on the recording medium. Then, the toner image fixed on this recording medium is exfoliated from the image carrier.
In accordance with this method, after cohesive power produced among the mutual toners which are penetrated into the recording medium becomes larger than adhesive power produced between the toners and the image carrier, since the toners are separated from the image carrier, a so-called xe2x80x9coffsetxe2x80x9d (namely, a portion of toners are left on image carrier) can be prevented, oilless toner separations can be realized, and furthermore, the transfer efficiency of these toners can be increased, so that the images having better color balance can be obtained. In addition, since the toners are made solid along the surface of the image carrier, such high-quality images having high-class feelings can be produced, while unform glossy feelings can be realized and superior transparent characteristics of the toners can be obtained.
However, in order to obtain such a high-quality image, even after the toner image has been simultaneously transferred/fixed to/on the recording medium, cooling operations of both the intermediate transfer member and the recording medium are required while this intermediate transfer member is made in close contact to the recording medium. When the recording medium is exfoliated from the intermediate transfer member before the toners sandwiched between the recording medium and the intermediate transfer member are cooled to be solidified, the toners of an exfoliated portion of this recording medium are not made solid along either the surface of the intermediate transfer member or the surface of the photosensitive drum, but concave/convex portions are formed. As a result, the glossy feelings are deteriorated. Therefore, there is a difference in the glossy feelings between the close-contacted portion and the exfoliated portion, and fluctuations in the glossy feelings may occur as the entire image, so that the image quality is considerably deteriorated. More specifically, in the case that a breathability of a recording medium is low, that is, an air permeability of a recording medium is high, for example, double-sided coated paper for printing, this exfoliation may occur. The reason why such an exfoliation occurs is given as follows. That is, water components contained in the coated paper are heated to be vaporized and expanded, and then, the vapor components pass through the coated layer to fall out from the coated paper. The water vapor which has been penetrated into the space in the side of the intermediate transfer member is stayed between this intermediate transfer member and the coated layer of the coated paper. Thus, the coated paper is eventually exfoliated from the intermediate transfer member by this water vapor pressure. This phenomenon will be referred to as an xe2x80x9cexfoliation blisterxe2x80x9d hereinafter.
FIG. 1 is a diagram for illustratively showing an occurrence condition of an exfoliation blister.
In FIG. 1, four sets of rectangle-shaped toner images 110 are arranged in such a manner that these four toner images 110 are separated from each other in a slight interval. An exfoliation blister 111 occurs while a non-image portion (namely, paper portion) corresponding to a center portion of the four arranged toner images 110 is positioned as a center.
FIG. 2 and FIG. 3 are diagrams for illustratively showing a mechanism by which the exfoliation blister occurs.
As indicated in FIG. 2, while paper 112 is overlapped with an intermediate transfer belt 113 which carries thereon the toner image 110, heat and pressure are applied to this paper 112 in a heat/pressure-applying unit (not shown). As a result, water components 114 contained inside this paper 112 are heated to be vaporized.
As indicated in FIG. 3, vaporized water components (namely, water vapor) 114a will fall out from the internal portion of this paper 112 to an external area. At this time, when this paper 112 corresponds to such a paper whose air permeability is small and which can easily penetrate therethrough air, the vaporized water components 114a may fall out from the rear surface of this paper. However, in the case of such a paper whose air permeability is large, for example, double-sided coated paper for printing, water vapor 114a may relatively gently fall out from the both surfaces of this paper 112. As a result, such water vapor 114a which has fallen out on the side of the intermediate transfer belt 113 is stayed at a region where the toner image 110 is not formed. Then, this water vapor pressure is increased, and while the paper 112 is deformed, the paper 112 is exfoliated from the intermediate transfer belt 113, so that a so-called xe2x80x9cexfoliation blisterxe2x80x9d 111 may occur.
A phenomenon called as a xe2x80x9cpaper blisterxe2x80x9d which is resembled to the above-described exfoliation blister 111 is such a phenomenon which may occasionally occur in the conventional printing systems and the fixing systems, which have been utilized many times. That is, in this paper blister phenomenon, since the water vapor 114a which has been enclosed in the coated layer of the coated paper is stayed within the coated layer, the interior portion of the paper will blister, so that paper fiber may be destroyed.
With respect to this paper blister phenomenon, various conventional techniques capable of adjusting air permeabilities of paper itself have been proposed. That is, the averaged center line roughness of the surface of the coated layer is adjusted smaller than, or equal to 20 xcexcm, and further, the air permeability is adjusted shorter than, or equal to 4000 seconds (see JP-A-62-198876, and JP-B-5-82940). Also, the coated layer having the better water vapor permeability, the coated amount of which is selected to be 2 to 5 (g/m2), is provided on both surfaces of the original paper having the specific air permeability, while the water vapor transmission ratio has been adjusted within the range between 50 and 500 (g/m2xc3x9724 hours) (see JP-A-1-245265).
However, the exfoliation blister may very easily occur, as compared with the paper blister. Thus, even when such a coated paper is employed whose air permeability has been adjusted so as to suppress an occurrence of a paper blister phenomenon, an exfoliation blister phenomenon may occur. Also, very recently, even in electro-photographic type image forming apparatus, no-dependable characteristics of paper sorts are required. More specifically, in order to satisfy needs made in a light printing field, for instance, on-demand publishing, adaptability characteristics of such electro-photographic type image forming apparatus to double-sided coated paper are necessarily required. The above-described solving means could not constitute such a proper solving means capable of solving the occurrence of this exfoliation blister phenomenon.
On the other hand, JP-A-9-330006 discloses the following technique. That is to say, both the recording medium and the image carrier which sandwich the toner image and to which heat and pressure have applied are cooled in such a way that these recording medium and image carrier are solidified on the cooling member whose surface is curved in the convex form. Furthermore, both the recording medium and the image carrier are pressure-depressed by the depressing means arranged opposite to this cooling member, so that the image carrier and the recording medium which sandwich the toner image are made in close contact to each other, and also both this image carrier and this recording medium which are brought into such a close contact condition are cooled so as to make the toner image solid. As a consequence, this conventional technique is capable of preventing deviation and exfoliation between the image carrier and the recording medium, or is capable of avoiding a floating phenomenon of the recording medium. Moreover, this conventional technique is capable of avoiding occurrences of offset, image fluctuations, and glossy fluctuations.
In accordance with this structure, such a mechanical deformation as the deviation, exfoliation, and floating between the image carrier and the recording medium can be prevented.
However, since an occurrence of such an exfoliation phenomenon is caused by vaporization of water components contained in paper, the temperature of this paper is high just after this paper has passed through the heat/pressure applying unit, and thus, a relatively large amount of water vapor existed between the image carrier and the recording medium may fall out from the paper, so that this paper may be deformed. As a result, when this paper reaches-the depressing member which is arranged opposite to the cooling member and is disclosed in JP-A-9-330006, a very large amount of water vapor has already fallen out and the paper has been deformed. As a consequence, it is practically difficult that the water vapor which has already fallen out is returned to this paper, the deformed paper is corrected, and further the contact condition between the image carrier and the recording medium is returned to the close contact condition. In other words, in order that the contact condition between the image carrier and the recording medium is returned to the close contact condition, sufficiently strong depression force is required. Since the depression operation by the depressing member is carried out against the cooling member which is solidified on the image carrier, if the sufficiently strong depression force is applied to both the recording medium and the image carrier, the load becomes excessively large. As a consequence, there are such problems that the drive control operation can be hardly carried out, and also the image carrier is worn away, which may impede long lifetime of this image carrier.
To preventing the occurrence of the exfoliation blister, while avoiding this problem, JP-A-2001-166610 opens such a technique that both the image carrier and the recording medium are sandwiched by the depressing unit constituted by the two roller-shaped members which are arranged on the upper stream side of the cooling member so as to return the contact condition between the image carrier and the recording medium to the close contact condition.
In accordance with these conventional techniques, since the depression operation is performed against the follow-actuated counter member, even in such a case that pressure is applied which is required to return the contact condition to the close contact condition, it is possible to avoid such a problem that the drive control operation cannot be carried out and the image carrier is worn away.
However, these technical ideas can hardly return a large amount of water vapor which has once fallen out to the paper, and can hardly return the contact condition to the close contact condition after the deformation of this paper has been reformed. Even if the contact condition can be returned to the close contact condition, then there is a positional shift between the original close-contact position and the close-contact position where the paper has been once exfoliated from the original close-contact position and thereafter this paper is again made in the close contact therewith. As a result, there are other problems that images are positionally shifted, and the deformed trail of this paper is lefted. Also, in such a case that the temperatures of both the image carrier and the recording medium are not yet decreased just after the recording medium has passed through the depressing unit, another exfoliation blister caused by water vapor may again occur after the paper has passed this depressing unit.
On the other hand, JP-A-5-142959 discloses the below-mentioned technique related to the belt fixing device. That is, the paper which carries thereon the unfixed toner image is sandwiched from both the front surface and the rear surface by the endless belt containing both the heat-applying roller and the exfoliating roller for the front surface side, and also the endless belt containing both the pressure-applying roller and the exfoliating roller for the rear surface side, and then, the paper is overlapped with these endless belts. Both heat and pressure are applied to the toner image by both the heat-applying roller and the pressure-applying roller. Then, the toner image is cooled under such a condition that this toner image is overlapped on the endless belts so as to be fixed on the paper. The toner image which has been fixed from the endless belts to the paper is exfoliated at the positions of the two exfoliation rollers located opposite to each other, so that the highly gloss images may be obtained with respect to both the front surface and the rear surface in the case of the double-plane (dual-plane) printing. In accordance with this disclosed technique, the member having the curve xe2x80x9cRxe2x80x9d abuts from one of the endless belts under overlapped condition at the position from the heat/pressure-applying position up-to the exfoliating position, and the close contact condition is established between the recording medium and the fixing belts by way of tension of these endless belts, so that occurrences of paper wrinkles (cockles) can be prevented.
However, when information is print out on double-sided coated paper for printing by employing the belt fixing device disclosed in JP-A-5-142959, substantially no deformed trail of this double-sided coated paper which is caused by the exfoliation blister can be recognized, but glossy fluctuations still remain.
The reason why such a glossy fluctuation is still left may be conceived as follows: That is, just after the double-sided coated paper for printing has passed through the heat/pressure-applying unit, the water vapor may fall out into a space defined between the endless belt and the recording medium. The deformation of this double-sided coated paper may be suppressed to a minimum deformation amount due to rigid characteristics of the rear-surface-sided endless belt, and also the amount of water vapor which falls out is also limited. However, a slight space may be produced between both the front-surface-sided and rear-surface-sided endless belts, and the recording medium. As a consequence, while utilizing only such effects achieved by the tension of these endless belts, which are described in JP-A-5-142959, the water vapor which has fall out into the space cannot be sufficiently returned to the recording medium, the recording medium foliated from the endless belt cannot be sufficiently returned to the close contact condition, but also the glossy fluctuations formed in the toner image cannot be sufficiently connected.
In view of the above-described circumstance, an object of the present invention is to provide an image forming apparatus and a fixing apparatus in which a high quality image without exfoliation blister even when a recording medium having high air permeability such as double-sided coated paper for printing is used in a simultaneous image transferring/fixing type image forming apparatus using a belt-shaped image carrier or in a fixing unit using a belt-shape fixing member.
To achieve the above-described object, according to a first aspect of the invention, there is provided an image forming apparatus including an image carrying body for carrying a toner image thereon and moving in a predetermined travel direction, a heating unit for heating the image carrying body to melt the toner image on the image carrying body, a pressure unit for overlapping a recording medium with the image carrying body while the toner image melted by the heating unit is sandwiched between the recording medium and the image carrying body, the pressure unit for applying pressure at a pressure position to the recording medium and the image carrying body, which are overlapped with each other, a recording medium supporting member moving in the predetermined travel direction from the pressure position to an exfoliation position while supporting the recording medium overlapped with the image carrying body, and a depressing/close-contacting unit for sandwiching and depressing the recording medium supporting member and the image carrying body in a middle position between the pressure position and the exfoliation position to closely contact the recording medium with the image carrying body. After the pressure unit applies pressure, an image is formed on the recording medium by exfoliating the recording medium from the image carrying body at the exfoliation position.
Preferably, the recording medium supporting member is an endless-shaped flat belt and a modulus of rigidity per unit width of the endless-shaped flat belt is not smaller than 0.8xc3x9710xe2x88x926 kgm2.
Also preferably, the depressing/close-contacting unit sandwiches and depresses the recording medium supporting member and the image carrying body at a place where the recording medium has a temperature not higher than 125xc2x0 C. and the toner image sandwiched between the recording medium and the image carrying body has a viscosity coefficient not larger than 105 Paxc2x7s.
Also preferably, the depressing/close-contacting unit includes at least one depressing portion having a roll-shape depressing member and a facing member facing the depressing member. The depressing member sandwiches and depresses the recording medium supporting member and the image carrying body so that pressure applied to the recording medium supporting member and the image carrying body is not lower than 2xc3x97104 Pa.
To achieve the object of the invention, according to a second aspect of the invention, there is provided a fixing apparatus including a fixing belt moving in a predetermined direction, a heating/pressuring unit having a pair of rollers sandwiching the fixing belt and being opposed to each other to form a nip portion, the heating/pressuring unit for overlapping the recording medium with the fixing belt while sandwiching a toner image between the recording medium and the fixing body to sandwich the recording medium and the fixing belt by the nip portion and for heating and applying pressure to the recording medium and the fixing belt at a heating position, a recording medium supporting member moving in the predetermined direction from the pressure position to an exfoliation position while supporting the recording medium overlapped with the fixing belt, and a depressing/close-contacting unit for sandwiching and depressing the recording medium supporting member and the fixing belt in a middle position between the pressure position and the exfoliation position to closely contact the recording medium with the fixing belt. After the heating/pressuring unit heats and applies pressure, an image is formed on the recording medium by exfoliating the recording medium on which the toner image is fixed from the fixing belt at the exfoliation position.
Preferably, the recording medium supporting member is an endless-shaped flat belt and a modulus of rigidity per unit width of the endless-shaped flat belt is not smaller than 0.8xc3x9710xe2x88x926 kgm2.
Also preferably, the depressing/close-contacting unit sandwiches and depresses the recording medium supporting member and the fixing belt at a place where the recording medium has a temperature not higher than 125xc2x0 C. and the toner image sandwiched between the recording medium and the fixing belt has a viscosity coefficient not larger than 105 Paxc2x7s.
Also preferably, the depressing/close-contacting unit includes at least one depressing portion having a roll-shape depressing member and a facing member facing the depressing member. The depressing member sandwiches and depresses the recording medium supporting member and the fixing belt so that pressure applied to the recording medium supporting member and the fixing belt is not lower than 2xc3x97104 Pa.